This invention relates to a downhole seismic energy source for use in geophysical prospecting while drilling, and more particularly to a seismic source having its frequency controlled by pipe rotation rate.
In modern rotary drilling systems, downhole acoustic energy generators have been used for various purposes. A U.S. Pat. No. 3,309,656 issued to J. K. Godbey on Mar. 14, 1967, for a Logging-While-Drilling System illustrates one type of acoustic generator used for telemetering information from the bottom of the borehole to the surface through the mud column within the drill string. The transducer employed by Godbey is fully described in column 5 of his patent as a fluid dynamic transducer or, in more common terms, a siren. The basic element of this type of transducer is simply a valve which repetitively opens and closes to block or allow the flow of a fluid, which in this case is drilling mud.
U.S. Pat. No. 3,860,902 issued to E. M. Galle on Jan. 14, 1975, for a Logging Method and System illustrates a second basic type of transducer also used for telemetering information to the surface. The Galle transducer is a fluidic oscillator having no mechanical parts which move at the oscillation frequency. In this patent, the fluidic oscillator resonant frequency is in part determined by the borehole itself so that the frequency which is detected at the surface is an indication of some borehole characteristics. This same fluidic oscillator is also used in U.S. Pat. No. 3,876,016 issued to L. B. Stinson on Apr. 8, 1975, for a Method and System for Determining the Position of an Acoustic Generator in a Borehole. The Stinson disclosure is essentially a method of determining the deviation of a borehole from vertical by detecting at various points on the earth's surface the seismic signals generated by a downhole seismic source.
Geophysical prospecting has usually involved the generation of acoustic waves of the earth's surface and the detection at the earth's surface of reflections of these same waves from subsurface formations. Considerable energy is lost when the waves pass through the upper surface layers of the earth. This surface attenuation can be avoided by placing a seismic source in a borehole. As illustrated by the above-referenced patents, seismic sources have been used in boreholes but not for geophysical prospecting. While the Stinson disclosure involves the detection of seismic energy at the earth's surface, the detected waves have traveled directly from the source, through the earth, to the detector and have not been attenuated by a reflection at a subsurface interface. Borehole dimensions limit the size of a seismic source which may be placed down a borehole and, therefore, limits the amount of energy which may be contained in a seismic initiation. This problem of low-peak power initiations can be overcome if a low-peak energy frequency-swept signal of long time duration is used. This type of signal is commonly used in geophysical prospecting systems bearing the trademark Vibroseis, which is a registered trademark of the Continental Oil Company. These initiations must be of known wave shape to be of value since they are detected by cross-correlation techniques. The initiations must also be repeatable since a standard way of increasing signal level is to repeat a given initiation on the order of ten times, while summing the seismometer responses to each initiation. The presently available downhole seismic sources do not meet these two requirements for Vibroseis type seismic exploration.